Machinery employed in the excavation, mining, cutting, working, or drilling of rock, stone, concrete and similar hard materials employ a variety of tools, hereinafter collectively referred to as “cutting tools”. Three commonly used types of cutting tools are picks, saws and drills.
Picks
Picks are used as cutting tools in machinery used in such applications as the mining of coal and the tunnelling through of rock. The term “pick” (also called “drag-tool”) typically means a pointed or chisel shaped rock cutting tool which cuts rock by penetrating and scraping along the surface of the rock. Picks typically consist of a steel shank with a tungsten carbide-cobalt material forming the cutting tip. This process produces relatively large rock fragments (or “cuttings”) as compared with the finer cuttings formed using tools having tips made from diamond or polycrystalline diamond composite (PDC).
Currently, the cutting head of a piece of mining or tunnelling machinery is fitted with a number of tool holders for orientating the cutting tools at a desired angle for striking the rock (the “angle of attack”). The cutting tools are “laced”, i.e. arranged in a pattern designed to effect relieved cutting, wherein as the cutting head rotates, each cutting tool has its work facilitated by the action of tools that it follows and, similarly, facilitates the work of each tool that follows it. This process allows rock fragments to be broken free with less energy than would be required if each tool had to excavate undamaged rock by unrelieved cutting.
Conventional picks, as previously stated, typically have a cutting tip formed from a tungsten-carbide-cobalt composite. These picks have a number of disadvantages.
Principally, tungsten carbide wears quickly when used to cut abrasive rock. Pointed tungsten carbide tips are designed to rotate in their holders during use so as to evenly distribute the wear. In practice, most tips do not rotate, resulting in the formation of wear flat. Even tips which do rotate as intended wear to a cone which contacts the rock surface along a line rather than at a point, thereby requiring much larger forces to fracture the rock compared to when the tip was new. Because of this wear, tungsten carbide tips can only effectively be used for cutting coal or soft rock. Accordingly, the average life span of a tungsten carbide tip is short and it must be replaced frequently.
There is clearly a need for a pick which has an increased life span, maintains a pointed shape throughout its use and which is strong and wear resistant enough to cut hard rock, such as granite.
Saws
Existing equipment for the cutting by sawing of rock, stone or concrete largely comprises impregnated diamond saw wheels and rock wheels.
Rock wheels are large wheels having pointed tungsten carbide tipped cutting elements, called “drag bits”, which remove rock in a chipping action. Due to the wear characteristics of the tungsten carbide tips, rock wheels are limited to use on rocks having a strength limit of about 100 to 120 MPa, such as sandstones. Accordingly, while they can be quite successfully used on soft rocks, rock wheels cannot be used on harder rock, such as granite.
Impregnated diamond saw wheels include as cutting elements peripheral segments of metal matrix composite material containing diamond grit. The sawing action is achieved by the scraping against the rock of the tiny protruding diamond particles which causes microfracturing. With each pass of the saw, only a very small amount of rock, e.g. a few microns, is removed as very small fragments. While such saws can be used to cut hard rock, the sawing process is very energy intensive and very slow.
There is clearly a need for a saw which can be used to cut hard rock, but wears at a slower rate than prior art tungsten carbide rock wheels, but saws at a faster and more energy efficient rate than prior art impregnated diamond saw wheels.
Drills
The drilling of soft rocks (e.g. coal, sandstone) is conventionally performed using drill bits incorporating largely pointed or chisel shaped tungsten carbide cutting elements. Cutting elements of such shape are termed “drag bits” in the art. These drag bits operate using a “chipping” action, removing a relatively large amount of rock as fragments at each pass, and so drill rapidly. However, due to the rapid wear of the tungsten carbide, these drill bits are not practical for use in drilling hard rock, such as granite.
Attempts have been made to produce tungsten carbide tool tips in which a very thin layer of diamond is grown over the tungsten carbide. However, such attempts have been unsuccessful due to distortion of tungsten carbide or decomposition of diamond at high temperatures.
Much of the drilling done in strong (hard) rock is currently effected using drill bits incorporating the relatively harder materials, diamond or polycrystalline diamond compact (PDC).
Diamond impregnated bits comprise diamond fragments embedded in a metal matrix composite (MMC) material. Diamond set bits comprise relatively larger natural diamonds mounted in MMC.
Alternatively, some drilling of hard rock is done using drill bits incorporating polycrystalline diamond compact (PDC) or thermally stable PDC. These drill bits comprise discs of the PDC mounted on a tungsten carbide-cobalt composite such that the edges of the discs scrape against the rock.
In all prior art drill bits which incorporate diamond or PDC as cutting elements, the cutting of the rock is effected by scraping the cutting element across the surface of the rock. Each pass causes microfracturing and removes a very small amount of rock, typically less than {fraction (1/10)} mm per pass. The rock is removed as tiny fragments, a process which is very energy intensive. The drilling process is accordingly slow, given the small amount of rock removed at each pass, and results in a drilling rate of only a meter or so per hour.
There is clearly a need for a drill bit for drilling hard rock which is strong and wears at a slower rate than prior art tungsten carbide bits, but operates more rapidly and efficiently than prior art diamond or PDC containing bits.
There have been numerous attempts to manufacture cutting tools having tips made from diamond or polycrystalline diamond composite (PDC) materials, with little success.
The present inventors have recognised that the inefficiency of prior art diamond or PDC containing cutting tools resides at least partially in the failure to provide such materials in the form of pointed or chisel shaped cutting bodies termed in the art as “drag bits”. Pointed bodies are able to press into the rock surface and remove rock as relatively large fragments which requires less specific energy with each pass than that required by prior art drag bits which scrape against the rock surface producing much smaller fragments. Furthermore, pointed bodies remove more rock with each pass, which results in a more rapid cutting process.
Diamond containing materials have typically been available in only a very limited range of shapes due to limitations of the moulding and machining processes used. Those shapes are triangles, squares, rectangles and half cylinders as cut from discs and cylinders by either laser cutting or electric discharge machining (EDM). It has not been possible to produce by direct synthesis pointed bodies, such as cones.
New generation diamond composite materials have been developed with properties superior to prior art composite materials. Such materials are termed “advanced diamond composites” (“ADC”) and are described, for example, in WO88/07409 and WO90/01986, the disclosures of which are incorporated herein by reference.
The ADC are typically formed by mixtures of diamond crystals and silicon to high pressures and temperatures to cause melting of the silicon which infiltrates between diamond particles and reacts with carbon of the diamonds to form silicon carbide. The silicon carbide forms a strong bond between the diamond crystals.
The diamond-silicon mixture may be placed adjacent silicon bodies during the reaction in order to enhance the infiltration of silicon into the mixture. This modification, which is the subject of WO88/07409, minimises detrimental porosity and microcracking and increases density, and thereby enhances the mechanical properties of the ADC.
In another modification, which is described in WO90/01986, a nitrogen and/or phosphorous containing material is introduced into the diamond-silicon mixture and/or the silicon bodies (if used) prior to reaction, such that the resulting silicon carbide bond in the ADC contains greater than a threshold amount of nitrogen and/or phosphorus. This threshold amount is typically 500 parts per million. The ADC product has low electrical resistivity—typically less than 0.2 ohm cm. A low electrical resistivity is advantageous in that it enables the shaping, working and machining of the ADC bodies by Electrical Discharge Machining (“EDM”)—also termed “wire-cutting” or “spark erosion”. EDM is far more versatile than conventional shaping techniques, such as laser cutting, both in terms of the size of bodies worked and the ranges of shapes able to be produced.
It has been found possible to mould and/or machine these ADC materials into a variety of shapes, including pointed bodies such as cones and bullet or ogival shaped bodies.
Although it is now possible to produce an effective shape using ADC materials, a further problem has been encountered, namely a means of effectively attaching the ADC bodies to tool bodies. Tool bodies are typically manufactured from steel, although they may include tungsten carbide components. The inventors have found that conventional methods of attaching the cutting tips to the tool body, such as by vacuum brazing, do not always provide a strong enough bond and the tips can accordingly break off during use. The inventors have surprisingly discovered that using a metal matrix composite to bond the cutting tip to the tool body produces a very strong and effective bond.